Improve Mixins page (#285)

* Improve Mixins page

State that:

1. Mixins are a __simple__ way to reuse component logic, rather than a flexible way to do it. The term __flexible__ goes into conflict with the last chapter of the page, which describes that mixins are in fact inflexible.
2. Add a section title for Precautions.

* Update src/guide/mixins.md

Co-authored-by: Stanislav Lashmanov <stasvarenkin@gmail.com>

Co-authored-by: Natalia Tepluhina <tarya.se@gmail.com>

Author: CyberAP

src/guide/mixins.md

@@ -2,7 +2,7 @@
 
 ## Basics
 
-Mixins are a flexible way to distribute reusable functionalities for Vue components. A mixin object can contain any component options. When a component uses a mixin, all options in the mixin will be "mixed" into the component's own options.
+Mixins distribute reusable functionalities for Vue components. A mixin object can contain any component options. When a component uses a mixin, all options in the mixin will be "mixed" into the component's own options.
 
 Example:
 
@@ -213,6 +213,8 @@ app.mixin({
 })
 ```
 
+## Precautions
+
 In Vue 2, mixins were the primary tool to abstract parts of component logic into reusable chunks. However, they have a few issues:
 
 - Mixins are conflict-prone: Since properties from each feature are merged into the same component, you still have to know about every other feature to avoid property name conflicts and for debugging.